I got an unusual Faber-Castell on eBay. It looks like a button fill Rapidograph with a hard rubber blind cap and rubber dome jewel. It is marked "Made in Germany". Anyway, I cleaned it up and got out the feed and nib to look into the ink system. It appears like it unscrews an internal sac nipple with a spanner wrench since I see a familiar looking cut across the opening. I worked in industrial assembly. Anyone ever work on one?